I've had depression for just over 4 years now. The first year or two I didn't notice much difference to my intelligence other than a bit of trouble concentrating, but I swear it's completely dumbed me down since then.
For starters I do everything more slowly - thinking, reading and talking - so absorbing information takes a lot longer. And everything is just so much harder to understand, although maybe that can be explained by lack of concentration (which has got way worse over the years).
My memory is pretty bad these days as well - at college I have to rewrite my notes several times to be able to retain information for even a little while, whereas a few years ago I could just hear it once and that's it, I'd remember it for ages.
Is this down to the depression? I believe it is because how smart I am is basically inversely proportional to how depressed I am, but I've never heard of these symptoms (other than trouble concentrating).
And if there are any whizzes out there, are there any studies showing this, and does it (my dumbness) go away when the depression does? I hope it does because I was actually pretty smart a few years back and now I find everything hard (cue violins).